Femtosecond pulse stretching fiber oscillator

ABSTRACT

A pulse stretching fiber oscillator (or laser cavity) may comprise a chirped fiber Bragg grating (CFBG) and an optical circulator arranged such that a first portion of a beam that is transmitted through the CFBG continues to propagate through the laser cavity while a second portion of the beam that is reflected from the CFBG is stretched and chirped by the CFBG and directed out of the laser cavity by the optical circulator. Accordingly, a configuration of the CFBG and the optical circulator in the laser cavity may enable pulse stretching contemporaneous with outcoupling, which may prevent deleterious nonlinear phase from accumulating prior to stretching.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/694,741, filed Nov. 25, 2019 (now U.S. Pat. No. 11,233,372), whichclaims the benefit of U.S. Provisional Patent Application No.62/866,377, entitled “FEMTOSECOND PULSE STRETCHING FIBER OSCILLATOR,”filed on Jun. 25, 2019, the contents of which are incorporated byreference herein in their entireties.

TECHNICAL FIELD

The present disclosure generally relates to a fiber oscillator, and moreparticularly, to a modelocked fiber laser cavity having a chirped fiberBragg grating and an optical circulator arranged to enable pulsestretching contemporaneous with outcoupling.

BACKGROUND

Modelocking refers to techniques in optics by which a laser isconfigured to produce ultrashort pulses that have a pulse duration onthe order of picoseconds (psec) or femtoseconds (fsec). Accordingly, amodelocked laser that is operated to produce ultrashort pulses issometimes referred to as a femtosecond laser and/or the like. Ingeneral, a modelocked laser is coupled to a laser cavity that contains amodelocking device (or modelocker), which may include an active elementsuch as an optical modulator, a nonlinear passive element such as asaturable absorber, and/or the like. The modelocking device causes anultrashort pulse to be formed, which circulates in the laser cavity. Ina steady state, effects that influence the circulating pulse are inbalance so that pulse parameters are unchanged after each completedround trip, or often even nearly constant throughout each round trip.Each time the pulse hits an output coupler mirror, a usable pulse isemitted, so that a regular pulse train leaves the laser. Assuming asingle circulating pulse, a pulse repetition period corresponds to around-trip time in the laser cavity (typically several nanoseconds),whereas the pulse duration is much shorter. Accordingly, a modelockedlaser can have a peak power orders of magnitude higher than an averagepower.

SUMMARY

According to some implementations, a pulse stretching laser cavity maycomprise: an active fiber configured to transmit a pulse, wherein thepulse propagates in a forward direction through the laser cavity andexperiences gain in the active fiber; an optical circulator thatcomprises an input port arranged to receive the pulse after the pulsepasses through the active fiber, a first output port, and a secondoutput port arranged to deliver an output pulse; and a chirped fiberBragg grating that comprises an input end arranged to receive the pulsefrom the first output port of the optical circulator, wherein thechirped fiber Bragg grating is configured to transmit a first portion ofthe pulse out a distal end of the chirped fiber Bragg grating where thefirst portion of the pulse continues to propagate in the forwarddirection to complete a round trip to the active fiber while a secondportion of the pulse is reflected and thereby stretched, and wherein thestretched second portion of the pulse propagates in a reverse directionback to the optical circulator where the stretched second portion of thepulse is diverted to the second output port.

According to some implementations, a pulse stretching laser cavity maycomprise: an active fiber configured to transmit a pulse, wherein thepulse propagates in a forward direction through the laser cavity andexperiences gain in a first pass through the active fiber; a chirpedfiber Bragg grating that comprises an input end arranged to receive thepulse after the pulses passes through the active fiber, wherein thechirped fiber Bragg grating is configured to transmit a first portion ofthe pulse out a distal end of the chirped fiber Bragg grating where thefirst portion of the pulse continues to propagate in the forwarddirection to complete a round trip to the active fiber while a secondportion of the pulse is reflected and thereby stretched, and wherein thestretched second portion of the pulse propagates in a reverse directionwhere the stretched second portion of the pulse experiences gain in asecond pass through the active fiber; and an optical circulator arrangedto receive the stretched second portion of the pulse after the secondpass through the active fiber and output the stretched second portion ofthe pulse.

According to some implementations, a pulse stretching laser cavity maycomprise: an active fiber; a reflective modelocker device arranged toreflect a pulse into the active fiber; an optical circulator thatcomprises an input port and a plurality of output ports, wherein theplurality of output ports comprise a first output port arranged toreceive the pulse after the pulse passes through the active fiber and totransmit the pulse via a second output port; and a chirped fiber Bragggrating that comprises an input end arranged to receive the pulse fromthe second output port of the optical circulator, wherein the chirpedfiber Bragg grating is configured to transmit a first portion of thepulse out a distal end of the chirped fiber Bragg grating and into theinput port of the optical circulator, and wherein the chirped fiberBragg grating is configured to reflect and thereby stretch a secondportion of the pulse back into the second output port of the opticalcirculator where the stretched second portion of the pulse is divertedto a third output port arranged to deliver an output pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example modelocked oscillator.

FIGS. 2A-2B are diagrams of one or more example implementations of amodelocked oscillator including a chirped fiber Bragg grating (CFBG) andan optical circulator arranged to enable pulse stretchingcontemporaneous with outcoupling in a ring cavity.

FIG. 3 is a diagram of an example implementation of a modelockedoscillator including a CFBG and an optical circulator arranged to enablepulse stretching contemporaneous with outcoupling in a linear cavity.

DETAILED DESCRIPTION

The following detailed description of example implementations refers tothe accompanying drawings. The same reference numbers in differentdrawings may identify the same or similar elements.

Short-pulse fiber oscillators typically operate with sufficiently highpeak powers that nonlinear effects are strong during pulse propagationin the fiber. While these nonlinearities are expected and typicallyuseful in generating short pulses by the oscillator, the nonlinearitiesmay be detrimental to overall system performance after the pulse isoutcoupled from the oscillator. For example, in femtosecond lasers usedin industrial, medical, and/or other applications, the pulse istypically stretched using a technique known as chirped-pulseamplification in order to reduce the peak power and the nonlinearitiesprior to launch into amplifiers that bring the pulse energy fromnanojoule (nJ) levels up to microjoule or millijoule levels. Thestretching technique can increase the pulse duration from less than 1picosecond (psec) or less than 10 psec up to a pulse duration greaterthan 100 psec or greater than 1 nanosecond (nsec).

Accordingly, the stretching technique can reduce the peak power byfactors from 10× to greater than 1000× and substantially reduce thenonlinearities. However, in systems that stretch a pulse using thechirped-pulse amplification technique or other similar techniques, aremaining length of optical fiber typically runs from the oscillatoroutput coupler to the pulse stretcher, and a substantial amount ofdetrimental nonlinear phase can be accumulated in the remaining lengthof the optical fiber that runs from the oscillator output coupler to thepulse stretcher. This nonlinearity can limit the power at which theoscillator can be operated, can limit the final compressed output pulseduration, and/or the like. In general, the level of nonlinearity may becharacterized by a B-integral, which for a length L of passive silicafiber is given by:

B=2πn ₂ I L/λ

where n₂=3×10⁻²⁰ m²/W, I is an intensity of a light in the fiber core,and λ is a wavelength. For example, the above expression can be used tocalculate that 1 kW of 1030 nm light in a polarization-maintaining 980nanometer (PM-980) fiber results in a B-integral of about 2π, which isroughly the level where nonlinear effects upon the pulse may becomesignificant.

Accordingly, a typical soliton or quasi-soliton modelocked fiberoscillator may generally operate in a range of up to 0.1 to 1.0 nJenergy per pulse, with a pulse duration of 100 femtoseconds (fsec) to 1psec, thus with peak powers in a 1 kW regime. Typical similariton- orAll-Normal Dispersion (ANDi)-modelocked fiber oscillators intrinsicallygenerate a somewhat stretched (e.g., chirped) pulse, typically in arange from 1 psec to 10 psec, which may be compressible down into arange of less than 1 psec to enable operation with somewhat higher pulseenergies of 1.0 to 10 nJ per pulse while remaining in the 1 kW regime.If any of these lasers were to be operated at higher pulse energylevels, the nonlinearity would grow accordingly, and the output pulsesmay be degraded due to having to traverse an output fiber on an order oftypically 1 meter or more prior to stretching. Accordingly, the growingnonlinearity may result in an unacceptable pulse-following recompression(e.g., a substantial pulse pedestal, a substantially longer pulseduration, a pulse broken up into multiple sub-pulses, and/or the like).

Some implementations described herein relate to a modelocked fiber lasercavity having a configuration in which a chirped fiber Bragg grating(CFBG) and an optical circulator are arranged to enable pulse stretchingcontemporaneous with outcoupling, thereby eliminating propagation of anunstretched pulse in an output fiber to a stretcher, the correspondingaccumulation of deleterious nonlinear phase prior to stretching, and/orthe like. Examples of modelocked fiber laser cavities in which a CFBGand an optical circulator are arranged to enable pulse stretchingcontemporaneous with outcoupling are described below.

FIG. 1 is a diagram of an example modelocked oscillator 100. Forexample, in some implementations, the modelocked oscillator 100 may be asoliton modelocked ring oscillator, a quasi-soliton modelocked ringoscillator, and/or the like. In some implementations, the modelockedoscillator 100 may be designed to operate polarized, using polarizing orpolarization-maintaining fiber and components. Additionally, oralternatively, in some implementations, the modelocked oscillator 100may be unpolarized.

As shown in FIG. 1, the modelocked oscillator 100 may include a pump 102and a pump wavelength division multiplexer (WDM) 104 configured togenerate energy that causes a pulse to be transmitted in a forwarddirection (e.g., clockwise) through a ring cavity. For example, thepulse may be generated spontaneously from noise in the ring cavity, andthe pulse may be shaped by one or more elements in the ring cavity asthe pulse makes round trips through the ring cavity. Accordingly, asshown in FIG. 1, the pulse may circulate in the clockwise directionaround the ring cavity and propagate through an active fiber 106 wherethe pulse experiences gain (e.g., is amplified). For example, the activefiber 106 may act as a gain medium to transmit the pulse, and mayinclude a glass fiber doped with rare earth ions such as erbium,neodymium, ytterbium, praseodymium, thulium, and/or the like.

As shown in FIG. 1, after passing through the active fiber 106, thepulse then passes through an output coupler 108 having a first outputport that couples onto an output fiber 110 and a second output port thatcouples into the ring cavity. Accordingly, after passing through theoutput coupler 108, the pulse may pass through a reflective modelockerdevice 114 (e.g., a semiconductor saturable absorber mirror (SESAM)) viaa first optical circulator 112, and through a double-pass dispersioncontrol device 118 via a second optical circulator 116 before returningto the active fiber 106. For example, as shown in FIG. 1, thedouble-pass dispersion control device 118 may include a lens 120, a pairof diffraction gratings 122, and a reflector 124 that are arranged toprovide a negative group-delay dispersion (GDD). In order to operate inthe soliton regime, a total GDD in the cavity should generally benegative or zero. Accordingly, the double-pass dispersion control device118 may be arranged to more than offset a positive GDD from an entirefiber length in the cavity to enable operation in the soliton regime. InFIG. 1, the output coupler 108 may be an all-fiber fused coupler, apigtailed partial reflector type device, and/or the like. In general,the output coupler 108 may operate according to one or more outputcoupling parameters that depend on specifics of a design of themodelocked oscillator 100. For example, in some implementations, theoutput coupler 108 may operate according to one or more output couplingparameters in a range from about 20% to about 80% output coupling. Intypical chirped-pulse amplification systems, the output pulses deliveredthrough the output fiber 110 are subsequently stretched in duration by apulse-stretching element 111 such as a CFBG, a volume Bragg grating, adiffraction-grating pair, and/or the like prior to pulse amplificationand compression.

As indicated above, FIG. 1 is provided merely as one or more examples.Other examples may differ from what is described with regard to FIG. 1.

FIGS. 2A-2B are diagrams of one or more example implementations 200 of amodelocked oscillator including a chirped fiber Bragg grating (CFBG) 202and an optical circulator 204 arranged to enable pulse stretchingcontemporaneous with outcoupling in a ring cavity. For example, exampleimplementation(s) 200 may include a pulse stretching fiber oscillator(or laser cavity) in which a pulse is spontaneously generated from noisein the laser cavity and shaped by one or more elements in the lasercavity (e.g., SESAM 114, dispersion control device 118, nonlinearity,and/or the like) as the pulse makes many round trips through the lasercavity. As shown in FIG. 2A, the pulse stretching fiber oscillator (orlaser cavity) includes a pump 102 and a pump WDM 104 arranged togenerate energy that causes the pulse to propagate into an active fiber106 of the laser cavity (e.g., a ring cavity in the illustratedexample), with the pulse propagating in a forward (e.g., clockwise)direction through the laser cavity and experiencing gain in the activefiber 106. Furthermore, as shown in FIG. 2A, the laser cavity includesthe CFBG 202 and the optical circulator 204 (e.g., in contrast to theoutput coupler 108 in the modelocked oscillator 100 of FIG. 1). In someimplementations, the optical circulator 204 includes an input portarranged to receive the pulse after the pulse passes through the activefiber 106, a first output port that couples into the laser cavity, and asecond output port that leads to an output fiber 110. Furthermore, asshown, the CFBG 202 includes an input end arranged to receive the pulsefrom the first output port of the optical circulator 204. In someimplementations, as described herein, the CFBG 202 may be configured totransmit a first portion of the pulse out a distal end of the CFBG 202and into the laser cavity where the first portion of the pulse continuesto propagate in the forward direction while a second portion of thepulse is reflected and thereby stretched. In some implementations, asfurther described herein, the second portion of the pulse that isreflected and thereby stretched propagates in a reverse (e.g.,counterclockwise) direction back to the optical circulator 204 where thesecond portion of the pulse is diverted to the second output port thatleads to the output fiber 110.

In general, a fiber Bragg grating (FBG) is a particular type ofdistributed Bragg reflector constructed in a short segment of opticalfiber to reflect particular wavelengths of light and transmit allothers. This effect is typically achieved by creating a periodicvariation in a refractive index of a fiber core, which generates awavelength-specific dielectric mirror. An FBG can therefore be used asan inline optical filter to block certain wavelengths, or as awavelength-specific reflector. The CFBG 202 is a specific type of FBG inwhich a grating that is inscribed in a fiber core has a non-uniformpitch, causing different spectral components of an input pulse to bereflected at different locations along the grating, resulting in GDD.For example, in some implementations, the non-uniform pitch of the CFBG202 may cause first and second spectral components of the second portionof the pulse (e.g. red and blue components) to be reflected at first andsecond locations along the CFBG 202. CFBGs are often used in areflective-only mode for pulse stretching, where the input pulse entersfrom one end, the reflected and stretched pulse exits from that sameend, and any unreflected light exits from the distal end of the fiberand is typically discarded. A circulator may be used at the input end toseparate the output light from the input (similar to the use of acirculator with the SESAM and the dispersion control device, asdescribed above with reference to FIG. 1). As shown in FIG. 2, the CFBG202 is incorporated into the ring cavity of the modelocked oscillator.

In some implementations, as shown in FIG. 2A, the CFBG 202 may bedesigned for a desired or optimal output coupling according to a designof the ring cavity of the modelocked oscillator. For example, in someimplementations, the CFBG 202 may be configured to reflect the secondportion of the pulse according to a reflectivity or outcoupling fractionthat is based on one or more output coupling parameters (e.g., fromabout 20% to about 80% output coupling). Furthermore, as shown in FIG.2A, the optical circulator 204 may be connected in the ring cavity in anorientation with the first output port (or output leg) coupling into thering cavity through the CFBG 202, and the second output port (or outputleg) coupling to the output fiber 110, which is opposite to typicalconfigurations in which the first output port leads to a reflectivedevice (e.g., a SESAM or a conventionally arranged CFBG) and the secondoutput port leads to an ongoing beam propagation direction. Further, adistal end of the CFBG 202 connects to the rest of the ring cavity,rather than being left unused as in a typical CFBG implementation.

For example, as shown in FIG. 2A, a beam propagation stage coupledbetween the active fiber 106 and the distal end of the CFBG 202 includesone or more devices arranged to propagate the first portion of the pulsein the forward (clockwise) direction from the distal end of the CFBG 202to the active fiber 106. For example, the one or more devices in thebeam propagation stage, which are configured in a similar manner asdescribed above with reference to FIG. 1, may include an opticalcirculator 112 that has an input port coupled to the distal end of theCFBG 202, a first output port that leads to a reflective modelockerdevice 114 (e.g., a SESAM), and a second output port that leads in anongoing beam propagation direction. In this example, the one or moredevices in the beam propagation stage further include the opticalcirculator 116 with an input port coupled to the second output port ofthe optical circulator 112, a first output port that leads to thedispersion control device 118, and a second output port that leads inthe ongoing beam propagation direction and towards the active fiber 106.

Accordingly, in operation, a clockwise-traveling pulse from the activefiber 106 may generally pass through the optical circulator 204 and outthe first output port to the CFBG 202, where some of the pulse istransmitted out the distal end of the CFBG 202 unstretched and continuesthrough the cavity. Furthermore, some of the pulse is reflected by theCFBG 202 and is thereby stretched. This stretched pulse propagatesbackward to the optical circulator 204, where the stretched pulse isdiverted to the second output port, which leads into the output fiber110. In this way, because the output pulse is stretchedcontemporaneously with being separated from the forward-going pulse inthe CFBG 202, the output pulse does not propagate in fiber in anunstretched state, and peak powers of the output pulse are substantiallydecreased. In this way, little or no nonlinearity is experienced by theoutput pulse. Additionally, or alternatively, using the CFBG 202 and theoptical circulator 204 within the ring cavity of the modelockedoscillator reduces optical loss that is typically incurred with aconventional outcoupler and a standalone CFBG and circulator. In thisway, the oscillator and stretcher combination delivers higher outputpowers, reducing a need for additional amplification stages andimproving the temporal and/or spectral quality of the pulse, which leadsto higher output pulse energy, shorter pulses, superior pulse qualityfollowing amplification and compression, and/or the like.

In some implementations, the arrangement of the SESAM 114 and thedispersion control device 118 as shown in FIG. 2A may be replaced by anoptical assembly that provides dispersion control, modelocking, spectralfiltering, and/or the like in the laser cavity. For example, the opticalassembly may generally have a similar configuration as the dispersioncontrol device 118, and may further including a focusing optic (e.g., alens, a concave mirror, and/or the like) positioned between the pair ofdiffraction gratings 122 and the reflector 124, which may be amodelocking device such as a SESAM positioned at a reflective end of theoptical assembly. Accordingly, the focusing optic may create a beamwaist at the reflector 124 and cause the beam to be inverted on a secondpass through the pair of diffraction gratings 122. In this way, bycausing the beam to be inverted upon the second pass through the pair ofdiffraction gratings 122, the optical assembly may produce a temporallyand spatially dispersed output that provides spectral filtering inaddition to dispersion control and modelocking in the laser cavity.Accordingly, in this case, the SESAM 114 may be omitted and the focusingoptic may be included in the dispersion control device 118 between thepair of diffraction gratings 122 and the reflector 124 to providedispersion control, modelocking, spectral filtering, and/or the like.Furthermore, because the CFBG 202 may have lower transmissivity near thecenter wavelength than at the flanks of the laser spectral distribution,tending to drive the laser to lase away from the center wavelength, aspectral bandpass filter may be provided in the cavity to compensatethis transmissivity distribution of the CFBG 202. For example, in a ringcavity configuration (e.g., as illustrated in FIG. 2A), such a spectralbandpass filter can be a transmissive element, a reflective element witha circulator, an optical assembly that provides spectral filteringthrough spatial dispersion as described above, and/or the like.

FIG. 2B illustrates another possible design for the modelockedoscillator in which the CFBG 202 and the optical circulator 204 arearranged to enable an extra gain pass. For example, in FIG. 2B, theoptical circulator 204 is located in a position counterclockwise fromthe active fiber 106 and the pump WDM 104, which may improve performancerelative to the arrangement of the modelocked oscillator as shown inFIG. 2A.

Accordingly, in FIG. 2B, the optical circulator 204 used for outputcoupling is located in a position counterclockwise from the active fiber106 and the pump WDM 104 and clockwise from the beam propagation stagethat includes the optical circulator 112, reflective modelocker device114, optical circulator 116, and dispersion control device 118. As aresult, a stretched output pulse traveling counterclockwise from theCFBG 202 traverses the active fiber 106 before being outcoupled by theoptical circulator 204. Accordingly, in FIG. 2B, the CFBG 202 includesan input end arranged to receive the pulse after the pulses passesthrough the active fiber 106, and the CFBG 202 may transmit a firstportion of the pulse out a distal end of the CFBG 202 and continuingthrough the laser cavity where the first portion of the pulse continuesto propagate in the forward (e.g., clockwise) direction while a secondportion of the pulse is reflected and thereby stretched. In someimplementations, the stretched second portion of the pulse propagates ina reverse (e.g., counterclockwise) direction where the stretched secondportion of the pulse experiences further gain in a second pass throughthe active fiber 106, and the optical circulator 204 receives thestretched second portion of the pulse after the second pass through theactive fiber 106 and outputs the stretched second portion of the pulseonto the output fiber 110.

In this way, because the active fiber 106 is operating with asubstantial single-pass gain of typically 3-15 dB (e.g., 10 dB) in orderfor the modelocked oscillator to run above a threshold, the stretchedoutput pulse may experience a corresponding gain (e.g., 10 dB) prior toexiting the ring cavity. Thus, for example, a soliton oscillator may beable to generate stretched pulses compressible to 200 fsec with energy 2nJ instead of 0.2 nJ as in a conventional soliton oscillator limited bya 1 kW peak power. Furthermore, in this way, a gain experienced by thepulse from the active fiber 106 may not be high enough to reach athreshold of nonlinearity. In particular, the gain is on the order of10×, whereas the stretching is on the order of 1000×, whereby the peakpower is decreased ˜100× relative to a natural level in the cavity(which is near the threshold of nonlinearity). Furthermore, while thestretching effect is mathematically equivalent to dispersion, thestretching effect is substantially greater than the natural dispersionof fiber lengths of one or two meters. Accordingly, any fiber dispersionthat occurs in the modelocked oscillator shown in FIG. 2B or insubsequent stages of fiber amplification may simply add to the stretchedpulse duration (or subtract from the stretched pulse duration, dependingon the sign of the stretching and whether the subsequent stages of fiberamplification are operating in a normal or anomalous fiber dispersionregime). In any case, a final pulse compressor at the output end of thelaser system (e.g., coupled to the output fiber 110) can be adjusted toreduce the pulse duration such that the pulses would be nearlyFourier-transform-limited (e.g., the pulses may exhibit a shortestpossible pulse duration given a spectral bandwidth of the pulses).Because the output pulse is already stretched, the increased power isunlikely to cause significant added nonlinearity. In someimplementations, a power at which the pump 102 is operated may beincreased to account for this additional extraction from the activefiber 106. Other than that, the presence of thecounterclockwise-traveling stretched pulse may have little to no effecton normal modelocking behavior of the clockwise-traveling short pulse.Overall, the system may deliver greatly increased power levels withdecreased levels of nonlinearity compared to a typical oscillator.

Accordingly, whereas optical circulators are typically oriented in amanner whereby a middle leg goes to a component that is double-passed,some implementations described herein may arrange the optical circulator204 in an orientation for output coupling. Furthermore, while anintra-cavity CFBG 202 may be arranged to provide a small amount ofdispersion for an oscillating beam in some cases (similar to thediffraction grating pair 122), CFBGs are conventionally arranged toreflect the oscillator beam (e.g., as an end reflector of a linearcavity), with the transmitted beam potentially used as a (non-stretched)outcoupled beam. In contrast, some implementations described hereinprovide an arrangement in which the transmitted beam, which is nominallynot dispersed by the CFBG 202, remains in the cavity of the modelockedoscillator, while the reflected beam may be used as the output.Furthermore, dispersion of a “stretcher” CFBG, such as the one used insome implementations described herein, is substantially greater thanthat of an intra-cavity dispersing CFBG (e.g., stretching a 500 fsecpulse to a 200-500 psec pulse, as opposed to a few psec for anintra-cavity dispersing CFBG). The oscillator output is normally a fewpsec or less, and any major stretching for chirped-pulse amplificationoccurs separately. Accordingly, some implementations described hereinmay incorporate the pulse stretching into the cavity structure, whichmay eliminate propagation through a connecting fiber from oscillator tostretcher.

Furthermore, some implementations described herein may be useful forsoliton or quasi-soliton oscillators because soliton modelocking tendsto be limited in pulse energy due to a short, unchirped pulse output.Accordingly, some implementations described herein may combineadvantages of soliton modelocking (e.g., stability, robustness, a cleanpulse profile, and/or the like) with greatly increased output power.Furthermore, while soliton oscillators generally create pulses withsquared hyperbolic secant (sech²) temporal and spectral profiles, adifferent profile (e.g., a parabolic or quasi-parabolic stretched pulsetemporal profile) may be desired in some case. Accordingly, in someimplementations, pulse shaping can be achieved using an appropriatelytailored spectral filter or temporal modulator applied to an output beamin or after the second output leg of the optical circulator 204 used foroutcoupling in order to generate the desired temporal and/or spectralprofile. Additionally, or alternatively, such pulses can be generateddirectly using different types of fiber laser oscillators (e.g., asimilariton oscillator). Some implementations described herein may beapplied to such oscillators as well.

As indicated above, FIGS. 2A-2B are provided merely as one or moreexamples. Other examples may differ from what is described with regardto FIGS. 2A-2B. For example, while example implementation(s) 200 focusedon a pulse stretching design in which the CFBG 202 and the opticalcirculator 204 are arranged in a ring cavity, some implementations mayapply the pulse stretching design to linear cavities, hybrid ring andlinear cavities, and/or the like.

For example, FIG. 3 is a diagram of an example implementation 300 of amodelocked oscillator including a CFBG 302 and an optical circulator 304arranged to enable pulse stretching contemporaneous with outcoupling ina linear laser cavity. More particularly, as shown in FIG. 3, themodelocked oscillator may be configured as a pulse stretching fiberoscillator that includes a reflective modelocker device (e.g., a SESAM)306 arranged to reflect a pulse (e.g., a pulse that is spontaneouslygenerated from noise in the linear laser cavity) into an active fiber106 of the linear laser cavity. As further shown in FIG. 3, the opticalcirculator 304 is a four-port circulator that has an input port(labelled ‘1’) and a plurality of output ports. For example, the outputports include a first output port (labelled ‘2’) arranged to receive thepulse after the pulse passes through the active fiber 106 and totransmit the pulse via a second output port (labelled ‘3’). The CFBG 302may include an input end arranged to receive the pulse from the secondoutput port of the optical circulator 304, and the CFBG 302 may transmita first portion of the pulse out a distal end of the CFBG 302 and intothe input port of the optical circulator 304. Furthermore, the CFBG 302may reflect and thereby stretch a second portion of the pulse back intothe second output port of the optical circulator 304 where the stretchedsecond portion of the pulse is diverted to a third output port (labelled‘4’) that leads to the output fiber 110. In some implementations, thefirst portion of the pulse may be directed out of the first output portof the optical circulator 304 to make another pass through the activefiber 106 towards the reflective modelocker device 306.

Accordingly, in one round trip through the modelocked oscillator shownin FIG. 3, a pulse follows a path that starts from the reflectivemodelocker device 306. The pulse then passes through the active fiber106 and into the first output port (port 2) of the optical circulator304. The pulse is then transmitted to the second output port (port 3) ofthe optical circulator 304 and into the CFBG 302, and a first portion ofthe pulse transmitted through the CFBG 302 proceeds to the input port(port 1) of the optical circulator 304 where the first portion of thepulse is directed out of the first output port (port 2) to make anotherpass through the active fiber 106 and back to the reflective modelockerdevice 306. A second portion of the pulse that is reflected by the CFBG302 is stretched and coupled back into the second output port (port 3)of the optical circulator 304 and out of the third output port (port 4)of the optical circulator 304 to form a useful output pulse.

As indicated above, FIG. 3 is provided merely as one or more examples.Other examples may differ from what is described with regard to FIG. 3.

The foregoing disclosure provides illustration and description, but isnot intended to be exhaustive or to limit the implementations to theprecise forms disclosed. Modifications and variations may be made inlight of the above disclosure or may be acquired from practice of theimplementations.

Even though particular combinations of features are recited in theclaims and/or disclosed in the specification, these combinations are notintended to limit the disclosure of various implementations. In fact,many of these features may be combined in ways not specifically recitedin the claims and/or disclosed in the specification. Although eachdependent claim listed below may directly depend on only one claim, thedisclosure of various implementations includes each dependent claim incombination with every other claim in the claim set.

No element, act, or instruction used herein should be construed ascritical or essential unless explicitly described as such. Also, as usedherein, the articles “a” and “an” are intended to include one or moreitems, and may be used interchangeably with “one or more.” Further, asused herein, the article “the” is intended to include one or more itemsreferenced in connection with the article “the” and may be usedinterchangeably with “the one or more.” Furthermore, as used herein, theterm “set” is intended to include one or more items (e.g., relateditems, unrelated items, a combination of related and unrelated items,and/or the like), and may be used interchangeably with “one or more.”Where only one item is intended, the phrase “only one” or similarlanguage is used. Also, as used herein, the terms “has,” “have,”“having,” or the like are intended to be open-ended terms. Further, thephrase “based on” is intended to mean “based, at least in part, on”unless explicitly stated otherwise. Also, as used herein, the term “or”is intended to be inclusive when used in a series and may be usedinterchangeably with “and/or,” unless explicitly stated otherwise (e.g.,if used in combination with “either” or “only one of”).

What is claimed is:
 1. A pulse stretching laser cavity, comprising: anactive fiber configured to transmit a pulse, wherein the pulsepropagates in a forward direction through the pulse stretching lasercavity and experiences gain in a first pass through the active fiber; achirped fiber Bragg grating that comprises an input end arranged toreceive the pulse after the pulses passes through the active fiber,wherein the chirped fiber Bragg grating is configured to transmit afirst portion of the pulse out a distal end of the chirped fiber Bragggrating where the first portion of the pulse continues to propagate inthe forward direction to complete a round trip to the active fiber whilea second portion of the pulse is reflected and thereby stretched, andwherein the second portion of the pulse propagates in a reversedirection where the second portion of the pulse experiences gain in asecond pass through the active fiber; and an optical circulator arrangedto receive the second portion of the pulse after the second pass throughthe active fiber and output the second portion of the pulse.
 2. Thepulse stretching laser cavity of claim 1, further comprising: a beampropagation stage coupled between the optical circulator and the distalend of the chirped fiber Bragg grating, wherein the beam propagationstage comprises one or more devices arranged to propagate the firstportion of the pulse in the forward direction.
 3. The pulse stretchinglaser cavity of claim 2, wherein the optical circulator is arrangedbetween a pump wavelength division multiplexer and the beam propagationstage.
 4. The pulse stretching laser cavity of claim 1, wherein theoptical circulator is coupled to a pulse compressor configured to reducea duration of the second portion of the pulse.
 5. The pulse stretchinglaser cavity of claim 1, wherein the second portion of the pulse has oneor more of a squared hyperbolic secant or a parabolic temporal profile.6. The pulse stretching laser cavity of claim 1, wherein the secondportion of the pulse is reflected and thereby stretchedcontemporaneously with the chirped fiber Bragg grating separating thefirst portion of the pulse that continues to propagate in the forwarddirection.
 7. A laser cavity, comprising: an active fiber configured totransmit a pulse, wherein the pulse propagates in a forward directionthrough the laser cavity and experiences gain in a first pass throughthe active fiber; a pulse-stretching element that is configured toreceive the pulse after the pulses passes through the active fiber,wherein a first portion of the pulse continues to propagate in theforward direction to complete a round trip to the active fiber while asecond portion of the pulse is reflected and thereby stretched, andwherein the second portion of the pulse propagates in a reversedirection where the second portion of the pulse experiences gain in asecond pass through the active fiber; and an optical circulator arrangedto receive the second portion of the pulse after the second pass throughthe active fiber and output the second portion of the pulse.
 8. Thelaser cavity of claim 7, wherein the pulse-stretching element is a fiberBragg grating.
 9. The laser cavity of claim 7, further comprising: abeam propagation stage coupled between the optical circulator and thepulse-stretching element, wherein the beam propagation stage isconfigured to propagate the first portion of the pulse in the forwarddirection.
 10. The laser cavity of claim 9, wherein the beam propagationstage comprises one or more additional optical circulators, a reflectivemodelocker device, and a dispersion control device.
 11. The laser cavityof claim 7, further comprising: a pulse compressor coupled to theoptical circulator and configured to reduce a duration of the secondportion of the pulse.
 12. The laser cavity of claim 7, wherein thesecond portion of the pulse has squared hyperbolic secant temporal andspectral profiles.
 13. The laser cavity of claim 7, wherein thepulse-stretching element is configured to contemporaneously stretch thesecond portion of the pulse and allow the first portion of the pulse topropagate in the forward direction.
 14. A laser cavity, comprising: anactive fiber configured to transmit a pulse, wherein the pulsepropagates in a forward direction through the laser cavity andexperiences gain in a first pass through the active fiber; and apulse-stretching element that is configured to receive the pulse afterthe pulses passes through the active fiber, wherein a first portion ofthe pulse continues to propagate in the forward direction to complete around trip to the active fiber while a second portion of the pulse isstretched, and wherein the second portion of the pulse propagates in areverse direction where the second portion of the pulse experiences gainin a second pass through the active fiber and is thereafter output by anoptical circulator.
 15. The laser cavity of claim 14, wherein thepulse-stretching element is a chirped fiber Bragg grating.
 16. The lasercavity of claim 14, further comprising: a beam propagation stageconfigured to propagate the first portion of the pulse in the forwarddirection.
 17. The laser cavity of claim 14, further comprising at leastone of an additional optical circulator, a reflective modelocker device,or a dispersion control device.
 18. The laser cavity of claim 14,wherein the second portion of the pulse has one or more of a squaredhyperbolic secant, a parabolic temporal profile, or a quasi-parabolictemporal profile.
 19. The laser cavity of claim 14, wherein thepulse-stretching element is configured to contemporaneously stretch thesecond portion of the pulse and allow the first portion of the pulse topropagate in the forward direction.
 20. The laser cavity of claim 14,further comprising: the optical circulator.